=249. Classification of Methods.=—To detect the presence of potash in a mixture the aid of the spectroscope may be invoked. In the scale of the spectrum divided into 170 parts, on which the sodium line falls at 50, potassium gives three faint rather broad bands, two red, falling at 17 and 27, and one plum-colored band, near the extreme right of the spectrum, at 153. Potassium, however, does not give brilliant and well-marked spectral bands, such as are afforded by its associates rubidium, caesium, sodium, and lithium. A convenient qualitative test which, for practical purposes will be quite sufficient, may be secured by dipping a platinum loop into a strong acid solution of the supposed potash compound, and viewing through a piece of cobalt glass, the coloration produced thereby when held in the flame of a bunsen. The red-purple tint thereby produced should be compared with that coming from a pure potash salt similarly treated. If a fertilizer sample give no indication of potash when treated as above it may be safely concluded that it does not contain any weighable quantity of potash. For the estimation of the percentage of potash present in a given sample it may be safely assumed that all of value in agriculture will be given up to an aqueous or slightly acid solution if organic matter have been destroyed as indicated in a previous paragraph. In the case of minerals insoluble in a dilute acid the potash may be determined by some one of the processes given in the first volume.[200] The potash having been obtained in an aqueous or slightly acid (hydrochloric) solution, it may be determined either by precipitation as potassium platinochlorid or as potassium perchlorate. The former method is the one which has been almost exclusively used by analysis in the past, but the latter one is coming into prominence and by reason of the greater economy attending its practice and the excellent results obtained by some analysts, demands a generous consideration. =250. The Platinic Chlorid Method.=—The principle of this method rests on the great insolubility of the potassium platinochlorid in strong alcohol and the easy solubility of some of its commonly attending salts; _viz._, sodium, etc., in the same reagent. Before the precipitation of the potash it is necessary to remove the bases of the earths, sulfates, etc. Barium chlorid and hydroxid, ammonium oxalate or carbonate, sulfuric acid, etc., are used in conjunction or successively to effect these purposes in the manner hereinafter described. The filtrate and washings containing the potash are evaporated to dryness and gently ignited to expel excess of ammonium salts and in the residue taken up with water and acidulated with hydrochloric acid, the potash is precipitated with platinic chlorid solution. The best methods of executing the analysis follow.